Our previous studies identified a region on chromosome 4q that was linked to measures of insulin action. A candidate gene in this region is FABP2 which encodes the human intestinal fatty acid binding protein (IFABP). We identified a polymorphism in this gene which results in an alanine (Ala54) to threonine (Thr54) substitution at amino acid 54 of IFABP. We found a significant association between the Thr54-encoding IFABP genotype (frequency = 0.29) and increased fasting lipid oxidation rates and insulin resistance, and have further shown that recombinant Thr54 protein has a higher affinity for long-chain fatty acids as compared to recombinant Ala54 protein. We further investigated the physiologic consequences of the IFABP substitution, by analyzing fatty acid transport across permanently transfected cells expressing either Ala54 and Thr54 IFABP. We found that 3H lipid was transported at a faster rate across the Thr54-expressing cells as compared to the Ala54-expressing cells. To investigate the structural differences between Ala54 and Thr54 IFABP, we have collaborated with J. Hamilton and utilized 3-D NMR to solve the structure of both proteins when bound to long-chain fatty acids. We are currently analyzing the promoters of the IFABP gene from individuals who are homzygous for the Ala54 allele and are insulin sensitive and individuals who are homzygous for the Thr54 allele and are insulin resistant. We have identified several polymorphisms and two deletions which are in total linkage disequilibration with the Ala to Thr substitution. We plan to assay the functional consequences of these varied promoters by ligating them to a reporter gene and comparing their transcriptional activity in transfected cells.